In this paper, an energy harvesting architecture in an Underlay Cooperative Cognitive Network (UCCN) is investigated, in which power constrained Decode-and-Forward relays harvest energy from radio-frequency signals received from a source, and then consume the harvested energy by forwarding the recoded signals to their destination. These recoded signals are launched by a transmitting power which is the harvested energy per a time interval. Based on the energy harvesting architectures that have been studied, two operation protocols are proposed: UCCN with Power Splitting architecture (UCCN-PS), and UCCN with Time Switching architecture (UCCN-TS). The best cooperative relay in both protocols is taken to be the one that satisfies the following conditions: maximum harvested energy, and maximum decoding capacity. As a result of the best relay selection, the signal quality of the selected link from the best relay to the destination is enhanced by the maximum harvested energy. The system performance of the secondary network in the UCCN-PS and UCCN-TS protocols is analyzed and evaluated by the exact closed-form outage probabilities and throughput analyses over Rayleigh fading channels. The Monte Carlo simulation method is performed to verify the theoretical expressions. Evaluations based on outage probability and throughput show that the system performance of the secondary network in the UCCN-PS and UCCN-TS protocols improves when the number of cooperative relays and the interference constraint increase as well as when the primary receiver is farther from the transmitting nodes such as the source and relays of the secondary network. In addition, the throughput performance of the UCCN-PS protocol outperforms that of the UCCN-TS protocol. Finally, the effects of the power splitting ratio, energy harvesting time, energy conversion efficiency, target Signal-to-Noise Ratio (SNR), and location of cooperative relays on the system performance of the secondary network are presented and discussed.

Underlay cognitive radio (CR) permits unlicensed secondary users (SUs) to transmit their own data over the licensed spectrum unless the interference from the SUs on the licensed primary user (PU) exceeds an acceptable level. This paper proposes two generalized interference alignment (IA)-based distributed optimization designs for multiple secondary transceivers in the underlay CR case with channel uncertainty under assumption that the actual channel error norm is below a certain bound. One of the designs is an extension to an existing method and the other one is a new design. In these methods, the precoding and power allocation matrices for each SU are either independently or jointly optimized for imperfect channel knowledge to maximize the secondary rates and to hold the secondary interference on the primary receiver under an acceptable limit that is determined by the primary receiver. Numerical results prove the ability of the proposed methods to support significant secondary rates provided that the PU is protected from extra interference from SUs, even in presence of channel uncertainty.

Designing a medium access control (MAC) protocol is a key for implementing any practical wireless network. In general, a MAC protocol is responsible for coordinating users in accessing spectrum resources. Given that a user in cognitive radio(CR) networks do not have priority in accessing spectrum resources, MAC protocols have to perform dynamic spectrum access (DSA) functions, including spectrum sensing, spectrum access, spectrum allocation, spectrum sharing and spectrum mobility, beside conventional control procedure. As a result, designing MAC protocols for CR networks requires more complicated consideration than that needed for conventional/primary wireless network. In this paper, we focus on two major perspectives related to the design of a CR-MAC protocol: dynamic spectrum access functions and network infrastructure. Five DSA functions are reviewed from the point of view of MAC protocol design. In addition, some important factors related to the infrastructure of a CR network including network architecture, control channel management, the number of radios in the CR device and the number of transmission data channels are also discussed. The remaining challenges and open research issues are addressed for future research to aim at obtaining practical CR-MAC protocols.

We propose a primary traffic based multihop relaying algorithm with cooperative transmission (PTBMR-CT). It enlarges the hop transmission distances to reduce the number of cognitive relays on the route from the cognitive source (CS) to the cognitive destination (CD). In each hop, from the cognitive nodes in a specified area depending on whether the primary source (PS) transmits data to the primary destination (PD), the cognitive node that is farthest away from the cognitive relay that sends data is selected as the other one that receives data. However, when the PS is transmitting data to the PD, from the cognitive nodes in a specified area, another cognitive node is also selected and prepared to be the cognitive relay that receives data of cooperative transmission. Cooperative transmission is performed if the PS is still transmitting data to the PD when the cognitive relay that receives data of the next hop transmission is being searched. Simulation results show that the average number of cognitive relays is reduced by PTBMR-CT compared to conventional primary traffic based farthest neighbor relaying (PTBFNR), and PTBMR-CT outperforms conventional PTBFNR in terms of the average end-to-end reliability, the average end-to-end throughput, the average required transmission power of transmitting data from the CS to the CD, and the average end-to-end transmission latency.

In this paper, a hybrid overlay/underlay cognitive radio system is modeled as an M/M/1 queue where the rate of arrival and the service capacity are subject to Poisson alternations. Each packet as a customer arriving at the queue makes a decision to join the queue or not. Upon arrival, the individual decision of each packet is optimized based on his observation about the queue length and the state of system. It is shown that the individually optimal strategy for joining the queue is characterized by a threshold of queue length. Thus, the individual optimal threshold of queue length is analyzed in detail in this work.

In this letter, we address the performance analysis of underlay selective decode-and-forward (DF) relay networks in Rayleigh fading channels with non-necessarily identical fading parameters. In particular, a novel result on the outage probability of the considered system is presented. Monte Carlo simulations are performed to verify the correctness of our exact closed-form expression. Our proposed analysis can be adopted for various underlay spectrum sharing applications of cognitive DF relay networks.

Underlay cognitive systems allow secondary users (SUs) to access the licensed band allocated to primary users (PUs) for better spectrum utilization with the power constraint imposed on SUs such that their operation does not harm the normal communication of PUs. This constraint, which limits the coverage range of SUs, can be offset by relaying techniques that take advantage of shorter range communication for lower path loss. Symbol error rate (SER) analysis of underlay cognitive relay systems over fading channel has not been reported in the literature. This paper fills this gap. The derived SER expressions are validated by simulations and show that underlay cognitive relay systems suffer a high error floor for any modulation level.

In this letter, we consider a cognitive radio based multihop network under the spectrum sharing underlay paradigm. By taking into account the interference constraints, we present an exact closed-form expression for outage probability, which is valid for the whole signal-to-noise ratio regime. In addition, some numerical examples of interest that study the effect of the number of hops and/or the interferer threshold on primary users are illustrated and discussed. Numerical results show that multihop systems still offer a considerable gain as compared to direct transmission under the same limit of interference.

In this paper, we present a distributed and interactive admission and power control protocol for spectrum underlay environments. The protocol enables distributed primary users (PUs) to estimate and adjust the level of tolerable interference as their transmitting powers evolve to a given signal-to-interference-plus-noise ratio (SINR) target. The protocol also guides the powers of distributed secondary users (SUs) to achieve their own targets while restricting the transmitting powers from SUs so as not to interfere with the PUs. This restriction of interference from SUs to PUs is an essential part of cognitive radio networks (CRNs) and is facilitated by sending a warning tone from PUs to SUs in the proposed protocol. The SUs that have frequently received the warning tones turn off their transmitters and so autonomously drop from the system. This paper proves that, under the proposed interactive protocol, every PU finally achieves its target if it is originally feasible without SUs and the transmit powers of remaining SUs converge to a fixed point. The proposed method protects PUs perfectly in the sense that all the PUs reach their targets after power control. Numerical investigation shows how safely PUs are protected and how well SUs are admitted as a function of protocol parameters, the frequency of warning tones, the number of SUs to be admitted and the number of active PUs.

A hybrid overlay/underlay spectrum sharing method for cognitive radio networks based on user classification and convex optimization is proposed. Interference radii are configured for the primary receiver and each cognitive receiver. Cognitive users are divided into four groups and allocated different spectrum sharing patterns according to their distance from the primary transmitter and receiver. An optimal power allocation scheme that achieves the maximum sum rate of cognitive radio system on the premise of satisfying the interference constraint of primary receiver is acquired through the convex optimization method. Performance analysis and simulation results show that, compared with existing methods, our method leads to improved performance of achievable sum rate of cognitive users while guarantees the transmission of primary users.

The effect of a nonmagnetic hcp-underlayer on the epitaxial growth of CoCr19Pt10 magnetic layers on substrates of Al2O3(0001) single-crystal has been investigated. Thin films of (0001)-oriented single-crystal CoCr19Pt10 were obtained by employing non-magnetic underlayers of CoCr25Ru25 and CoCr25Ru25/Ti, while thin films of polycrystalline CoCr19Pt10 were grown after the deposition of underlayers of TiCr10 and CoCr40. The growth of thin film CoCr19Pt10 on a Ti(0001) underlayer was interpreted as quasi-hetero-epitaxial where the continuity of the lattice across the interface is disturbed while the overall crystallographic relationship between the two layers is maintained. A thin film of epitaxially grown CoCr19Pt10 has a compositional variation of a few percent across the film plane in terms of elements that forms the alloy.

The effects of an intermediate layer of carbon on the structural and magnetic properties of a CoCrPtTa recording layer were investigated in double-layered perpendicular magnetic recording media with either amorphous CoTaZr or crystalline FeAlSi as soft magnetic backlayers. Introducing a thin layer of carbon enhanced the perpendicular magnetic anisotropy with both soft magnetic backlayers. This result suggests that the introduction of a non-magnetic intermediate layer is useful in improving the basic magnetic properties of the CoCr-alloy recording layer even when an amorphous soft magnetic backlayers is used.

Microstructures of CoCr-alloy thin film media were investigated by cross-sectional transmission electron microscopy focussing on the initial growth region of the magnetic layer grown on nonmagnetic underlayers. An introduction of nonmagnetic hcp-CoCrRu layer between an hcp-CoCrPt recording layer and an hcp- or a bcc-underlayer improved the crystallographic quality of the initial growth region. Sharp compositional distributions of alloying elements at the interfaces of a CoCrPt/CoCrRu/CrTi perpendicular medium and a CoCrPt/CoCrRu/CrTi longitudinal medium were respectively confirmed by electron energy loss spectroscopy employing a finely focussed electron beam. Coercivity and squareness of the thin film media increased by realizing good hetero-epitaxy between the nonmagnetic and the magnetic hcp-layers.

Magnetoplumbite type Ba ferrite (BaM) disks were prepared on Pt and Pt-Ta underlayers using facing targets sputtering apparatus. Pt underlayers are more effective than ZnO underlayers to promote c-axis orientation of BaM layers, especially for extremely thin BaM films. Pt-Ta underlayer was used to decrease the grain size of BaM layers. BaM/Pt-Ta disks revealed larger S/N ratio than BaM/Pt disks because of their larger signal output and lower media noise level. BaM disks with 50 nm thick BaM layers revealed lower noise level and larger S/N ratio than that with 100 nm thick BaM layers due to smaller grain size.

With the rapid increase in recording density in recent years, the development of media with high coercivity and low noise has become an important issue in perpendicular as well as longitudinal recording. Compared with the CoCr binary system, the CoCrTa system is more effective in increasing coercivity. The increase in coercivity is due to not Ta segregation but to enhanced Cr segregation at column boundaries caused by Ta addition. When a Ti underlayer with uniform thickness was used, there was no improvement in c-axis alignment of the magnetic layers in CoCrTa and CoCrPt films, although the lattice mismatch of Co ternary alloys with Ti was much reduced compared with that of the CoCr film with Ti. This indicates that there may be no direct heteroepitaxial relationship between Ti underlayer and CoCr alloys at the Ti-Co alloy interface. In the case of the CoCrPt film, perpendicular coercivity increased linearly as Pt content increased up to 10at%. When Pt content was about 13at%, the shape of the M-H loop showed characteristics of domain wall motion reversal, which indicated strong exchange coupling among columns. When Ta content was increased to 4at% in the CoCrPt film, perpendicular coercivity increased and the shape of the M-H loop suggested that the domain wall motion reversal behavior was much reduced. This is thought to be associated with Cr segregation at column boundaries by Ta addition.

The authors have studied the demagnetization phenomenon which is observed in a conventional CoCrTa/CoZrNb double-layered (DL) perpendicular recording medium. The authors have also investigated the effects of an in-plane hard magnetic layer in a triple-layered (TL) perpendicular recording medium. The in-plane hard magnetic underlayer is made of CoSm or CoCrTa/Cr and is laid under the CoZrNb soft magnetic layer. In the DL medium, a demagnetization phenomenon i.e. decrease of the readback signal, is observed when the CoCrTa layer has a strong perpendicular orientation and the CoZrNb underlayer has a low coercivity. The amount of the signal decrease depends strongly on the accumulated disk revolutions. This demagnetization is considered to be caused by fact that the recorded magnetization in the CoCrTa layer is reduced by the magnetic field generated from the domain walls in the CoZrNb layer, since the CoZrNb layer is very sensitive to a magnetic environment such as geo-magnetism and domain walls move as the disk rotates. On the other hand in the TL medium, the hard magnetic layer has an effect of pinning the magnetic domain in the CoZrNb layer, by which the demagnetization problem is successfully prevented. The hard magnetic layer remarkably reduces the domain walls in the CoZrNb layer and contributes to medium noise reduction. Thus the TL medium presents a higher SN ratio than DL medium.

Ba ferrite films were deposited epitaxially on ZnO underlayer from targets with composition of BaO-6.5Fe2O3 at substrate temperature of 600 using the facing targets sputtering apparatus. The gas mixture of Ar and Xe of 0.18 Pa and O2 of 0.02 Pa was used as the sputtering gas and the dependences of crystallographic and magnetic characteristics on the partial Xe pressure PXe(0.0-0.18 Pa) were investigated. Films deposited at various PXe were composed of BaM ferrite and spinel crystallites, and the minimum centerline average roughness Ra of 8.3 nm was obtained at PXe of 0.10 Pa. Since saturation 4πMs of 5.1 kG and perpendicular anisotropy constant Ku1 of 4.23105 Jm-3 were larger than those of bulk BaM ferrite of 4.8 kG and 3.30105 Jm-3, respectively, these films appeared promising for use as perpendicular recording media.